Preheat oven to 200 degrees F. Slice apples on mandolin as thinly as possible. Place on a lined cookie sheet and sprinkle with cinnamon (optional). Bake for 1 hour, flip the slices, and bake an additional hour or until the slices are firm (mine took 2.5 hours total time to be crispy!). Allow to cool – they will crisp up a little more as they cool. Enjoy. Can be store in an airtight container for a few days.